1. Field of the Invention
The present invention relates generally to test fixture apparatus for electrically testing printed circuit boards and the like, and more particularly relates to apparatus and methods utilized to properly align the printed circuit board being tested with the test probe portion of the overall fixture structure.
2. Description of Related Art
The often complex internal circuitry in printed circuit boards and the like is conventionally tested by forming a spaced series of electrically conductive test contact points or "pads" on one side of the board, the contact points being operatively connected in a predetermined manner to the internal circuitry to be electrically tested for continuity and proper operation. The circuit board is positioned on a vertically movable top plate structure disposed within a test fixture, and is maintained in appropriate horizontal alignment with the top plate structure by means of a plurality of upwardly projecting tooling pins secured to the top plate and extending through corresponding alignment holes formed in the circuit board.
With the circuit board in place within the test fixture, a vacuum force is exerted on the board to downwardly move the top plate structure and draw the board into adjacency with a fixed probe structure having a spaced series of upwardly projecting resilient test pins, commonly referred to as "pogo" pins, which are intended to be in horizontal alignment with the board test contact points. As the circuit board under test reaches a lower limit position thereof within the test fixture, the pogo pins engage and are downwardly compressed by the test contact points. The pogo pins are appropriately connected to a suitable test and analysis circuit which operates to receive and analyze electrical test signals emanating from the contact points and indicative of the correctness and operativeness of the board's printed circuitry. After this testing is completed, the vacuum force is terminated and the top plate structure, along with the circuit board mounted thereon, is upwardly returned to its starting position, and the now-tested circuit board is removed and replaced with the next circuit board to be tested.
This conventional use of a vacuum driving force and the tooling pin circuit board mounting alignment method just described has proven to be quite rugged and reliable, providing satisfactory test results over the years in conjunction with printed circuit boards in a variety of sizes and circuitry configurations. However, as the test point "densities" of circuit boards (i.e., the number of test contact points per area unit) increase, and the sizes of the individual test contact points decrease, the suitability of each of these conventional fixturing movement and alignment techniques markedly diminishes.
For example, in modern high density circuit boards, it is not uncommon for their test contact points to be less than 10 mils in diameter and, in at least some locations on the board, be in extremely close proximity to one another. This high density contact point orientation, and miniaturization of the contact points themselves, significantly heightens the adverse effects of even rather minuscule misalignment between the test contact points and the associated probe structure with which they are to be brought into operative contact.
In the conventional circuit board alignment and movement method generally described above, this undesirable misalignment stems from two primary causes--the tooling pins and the vacuum force used to move the board under test downwardly toward the pogo pin-based probe structure. Specifically, after a great number of circuit boards are tested and removed from a particular test fixture, the tooling pins used to hold the boards in a precisely aligned orientation begin to wear along their side surfaces, resulting in a slightly looser fit between the pins and successive groups of boards being tested. Particularly in the case of high density circuit boards having tiny, closely spaced test contact points, even a slight amount of alignment "play" caused by eventual tooling pin side surface wear can cause various ones of the pogo pins to miss their intended contact point targets as the circuit board reaches the pins.
The vacuum force exerted on the circuit board to draw it downwardly toward the probe structure often tends to slightly bow the board, particularly in a central portion thereof, thus horizontally shifting some of the test contact points and compounding the misalignment problem caused by surface wear on the tooling pins. This misalignment of the test contact points tends to be even more serious in test fixtures designed to simultaneously engage test contact points on opposite sides of the circuit board being tested.
It can be readily seen from the foregoing that, particularly in the case of high test point density printed circuit boards, the provision of test fixture apparatus having improved circuit board alignment control capabilities would be highly desirable. It is accordingly an object of the present invention to provide improved test fixture apparatus, and related test methods, having such enhanced circuit board alignment control capabilities.